At first glance this looks easy. If spades behave, you have 10 tricks: six spades, two hearts and two clubs. But what if trumps divide poorly? If West holds trump length, you can’t do anything about it, but if East holds trump length, you can still bring home your contract.
Say this is the layout:
To succeed in picking up a long trump holding on your right, you must reduce your trump length to that of your opponent (you’ll see why in a minute). You’ve already ruffed once, so you must do so again.
Cash the trump ace and then play the ♣A K. When you cross to the trump king in dummy, West shows out. Ruff a club (making your trump length equal to RHO), then play three rounds of hearts. As long as East follows to two rounds of hearts, it doesn’t matter who wins the third round because your last two cards are the ♠Q 10, poised to capture RHO’s ♥ J 9.
This technique is called a trump coup.
To see the importance of reducing your trump length, try the play without ruffing a club and see what happens.